1. Field of the Invention
This invention relates to a process for continuously producing high-impact rubber modified styrene resins according to solution or bulk polymerization techniques. More particularly, it relates to improvements in the method of transforming a rubber-like polymer into dispersed particles.
2. Description of the Prior Art
Conventionally, it is a widespread practice to continuously produce rubber modified styrene resins in which a rubber-like polymer is dispersed in the form of finely divided particles having an average diameter of 0.1 to 10 microns. Since the size of the rubber-like polymer particles present in the product exerts a great influence on its performance characteristics such as impact strength, gloss and the like, the control of particle diameter occupies a very important position among the techniques involved in the production of rubber modified styrene resins.
In recent years, however, there are growing market demands for high-performance products responding to the extending range of use of rubber modified styrene resins and for cost reduction by the adoption of more efficient production processes. In order to meet these demands, it is necessary to solve the following problems concerning the transformation of a rubber-like polymer into dispersed particles in the continuous production of rubber modified styrene resins.
(1) It must be feasible in a single production system to selectively produce various grades of rubber modified styrene resins which differ in average particle diameter, rubber content and/or the type of rubber so as to meet the market demands for varying combinations of performance characteristics such as impact properties, surface gloss of molded articles, and the like.
(2) In order to improve the impact properties of rubber modified styrene resins and the gloss and other surface properties of molded articles, the formation of giant particles (observed as fisheyes) during the dispersing operation, which will be fully described later, must be prevented.
(3) In order to avoid cleaning operation of the reactor, the adhesion of rubber-like matter to the inner walls of the reactor during the dispersing operation must be prevented.
(4) In order to meet the demand for better impact properties of rubber modified styrene resins or compensate the impaired impact properties thereof due to the incorporation of a flame retardant and the like, it is required to produce rubber modified styrene resins containing a high concentration (e.g., not less than 10% by weight) of rubber. Accordingly, even when a feed material having a high rubber content (e.g., a feed material composed of 9-20% by weight of rubber and 91-80% by weight of styrene) is subjected to polymerization, it must be possible to transform the rubber-like polymer into dispersed particles.
Generally, the shape and size of the rubber-like polymer particles dispersed in a rubber modified styrene resin are controlled by the conditions under which the rubbery phase including a rubber-like polymer is changed from a continuous phase into a discontinuous one, that is, the conditions of the process of transforming the rubbery phase into dispersed particles (which process will hereinafter be referred to as the dispersing operation). In the continuous production of rubber modified styrene resins, this dispersing operation is carried out in parallel with the transferring and polymerizing operations of the reaction mixture, so that greater difficulties are encountered than in the continuous production of ordinary styrene resins or the batch production of rubber modified styrene resins. In order to overcome these difficulties, a number of processes requiring complicated procedures have heretofore been proposed.
One exemplary process is disclosed in U.S. Pat. No. 3,660,535. This process is characterized in that a portion of the reaction mixture is withdrawn from the tubular reactor and returned to a section thereof preceding the site of phase transformation. Japanese Patent Publication No. 29793/'77 has proposed another process which involves preliminary polymerization (referred to as pregrafting) prior to the dispersing operation. However, these processes do not offer a satisfactory solution of the above-described problems inherent in this field of art.